Periodic Classification of Elements: History & Periodic Trends-Class 11 Chemistry Notes

Periodic Classification of Elements refers to the arrangement of elements on the basis of the periodic repetition of their properties. It means the elements which exhibit similar properties on a regular interval are placed in the same group.

Classification of Elements

Elements are the basic units that make up all matter in our surroundings. Earlier, only 31 chemical elements were discovered in 1800. Around 63 new elements were discovered in 1865.

This necessitated the periodic classification of elements as it was difficult to remember the properties of each element. Hence, if we put all the alike elements in the same group we can predict the properties of each element of that group. There are presently 118 elements that we are aware of.

History of Classification of Elements

The Periodic Table which we learn today is not the result of a single effort. There have been several attempts in the past to classify elements, however, few got succeed and even these tables had some drawbacks and hence were revised by new classifications.

Hence, the Modern Periodic table evolved over the period. Some of the successful attempts to classify elements are listed below:

• Dobernier Triad
• Newland Law of Octaves
• Lother Meyer Arrangement
• Mendleeves Periodic Table
• Modern Periodic Table

Dobernier Triad

In 1829, Dobernier attempted the first-ever classification of elements. He placed 3 elements in a group and stated that the mass of the middle element is equal to the arithmetic mean of the other two elements. He made 3 such groups. The group formed by him is shown below:

In the first group, the atomic mass of Li is 7 and K is 39. If we find the mean of atomic masses of Li and K it will be (7 + 39)/2 = 46/2 = 23 which is the atomic mass of Sodium. This property was followed in the three groups formed.

Drawbacks of Dobernier’s Triad

The drawbacks of the Dobernier’s Triads are discussed below,

• Dobernier could only form three such triads out of 30 elements that were known at that time.
• Dobernier Triad was not applicable to very low or very high atomic mass elements
• In the case of F, Cl, and Br, the rule of arithmetic mean is not valid, although F, Cl, and Br are placed in the same order in the modern periodic table.

Newland Law of Octaves

Newland, a British Chemist made an attempt in 1865 to arrange atoms in increasing order of their atomic mass. He stated that when atoms are arranged on the basis of increasing order of the mass then every eighth element exhibits similar properties. This arrangement was based on his understanding of musical notes where the word repeats after every 7th interval. It is similar to Indian Musical Note Sa, Re, Ga, Ma…..

The elements arranged in Newlands law of Octaves are discussed in the image below,

Drawbacks of Newland Law of Octaves

The drawbacks of the Newland Law of Octaves are discussed below,

• It was applicable only to Calcium. After Calcium, every eighth element didn’t resemble the properties. 
• Dissimilar elements were placed in the same group while similar atoms were placed in different groups. For Example, Iron was placed with Oxygen and Sulphur.
• Some groups contain two elements in one place. For Example, Co & Ni.

Lother Meyer’s Arrangement

In 1869, Lother Meyer a German Chemist stated that when elements are arranged in order of increasing atomic mass then elements exhibit similar properties at regular intervals.

Mendleeve’s Periodic Table

Mendeleev in the same year 1869 unknown of the Lother Meyer’s arrangement gave the most successful and detailed periodic table. He stated that “The properties of elements whether physical or chemical are periodic functions of their atomic masses”. Based on his law he arranged all the attempts known at that time in a table called the Periodic Table, which later on became the basis of the arrangement of elements for the Modern Periodic Table. He called the vertical column ‘Groups’ and horizontal rows ‘Periods’.

Features of Mendeleeve’s Periodic Table

Important features of Mendeleeve’s Periodic Table are listed below:

• It was the most simplified arrangement of elements based on their properties. One can easily guess the property of elements placed in a particular group.
• He corrected the atomic masses of elements such as Beryllium, Gold, Indium, etc.
• He left vacant positions for some elements that were not known at that time but he predicted such elements would exist in the future. He added the prefix ‘eka’ to those elements. For Example, Eka Aluminium was Gallium, Eka-Silicon was Germanium.

Drawbacks of Mendeleeve’s Periodic Table

Following are the drawbacks of Mendeleeve’s Defect

• The position of Hydrogen was not defined. Hydrogen resembles properties with that of alkali metals as well as halogens but was placed in the group of alkali metals. He couldn’t explain why hydrogen will not be in the halogen group although both are nonmetals.
• There exists anomalous pair in the table which would not follow the law of increasing atomic weight. For Example, Argon (39.9 u) was placed before K (39.1) and Co was placed before Ni.
• Since the table was based on increasing order of atomic mass but the position of isotopes was not defined as isotopes have different masses.
• The position of Lanthanoids and Actinoids was not assigned separately.
• Similar atoms were placed in different groups while dissimilar atoms were placed in the same groups. For example, Alkali Metals Li, Na, and K were placed together with  Cu, Ag, and Au, even though their properties are quite dissimilar.

Modern Periodic Table

Modern Periodic Table is a long-form periodic classification of elements based on the electronic configuration of elements. Modern Periodic Table has vertical columns called ‘Groups’ and horizontal rows called ‘Periods’. There are 18 Groups and 7 Periods in Modern Periodic Table. The Modern Periodic Table was given by Henry Mosley in 1913.

Features of Modern Periodic Table

The following are the properties of the Modern Periodic Table:

• It is based on the atomic number of elements rather than atomic mass.
• Since it is based on atomic numbers hence it solved the problem of the position of isotopes.
• It allows systematic classification of Elements on the basis of s, p, d, and f on the basis of the subshell in which the last electron enters.
• In Modern Periodic Table Metals are on Left Hand Side while Non-Metals are on Left Hand Side. The two sides are connected by d-block elements also known as transition elements
• The elements which formed anomalous pairs in Mendeleeve’s Periodic Table got their justified position in this table as the classification is based on atomic number rather than atomic mass.
• Lanthanoid and Actinoids were placed in the table separately at the bottom.
• It accomodated much larger number of elements and even have spaces for elements which can be discovered in future. Presently there are 118 elements in the table.

Drawbacks of Modern Periodic Table

Although Modern Periodic Table solves all the issues of Mendeleeve’s classification yet it has some drawbacks:

• The position of Hydrogen is still not settled as Hydrogen resembles properties of both halogen and Alkali Metals.
• Lanthanoids and Actinoids are placed separately at the bottom and are not part of main table.

Modern Periodic Law

Modern Periodic Law became the basis of the Modern Periodic Table. It was given by Henry Mosely in 1913. It states that

Elements’ physical and chemical properties are periodic functions of their atomic numbers and not atomic mass.

Classification of the Elements in the Periodic Table

The elements in the periodic table can be classified in four ways based on their electronic configurations:

• Noble gas elements: Noble gases are elements in the modern periodic table’s group 18. The electronic configuration of this group’s first element (helium) is 1s². The rest of the elements (neon, argon, krypton, xenon, and radon) have an ns² np⁶ outer shell electronic configuration. Since the octet of these elements is complete, they are extremely stable.
• Representative elements: S-block and p-block elements are examples of representative elements. The elements in groups 1 and 2 are referred to as s – block elements (elements with 1s² and 2s² outermost configuration). Groups 13-17 are known as the p-block elements (outermost configuration varies from ns² np¹ to ns² np⁵).
• Transition elements: Transition elements are elements that belong to groups 3 to 12 and have an outer shell electronic configuration of (n-1)d^1-10 ns^1-2. These elements are also referred to as d-block elements.
• Inner transition elements: The inner transition elements are the lanthanides and actinides series, which are found at the bottom of the periodic table. The 4f and 5f orbitals are partially filled in these elements, giving them unique properties.
• Alkali and Alkaline Earth metals: The first two groups on the periodic table’s left side are made up of highly reactive elements (except hydrogen). The Elements of the first group are called Alkali Metals, while the elements of the second group are called Alkaline Earth Metals
• Metalloids and non-metals: Metalloids are typically found in a diagonal line on the periodic table’s right side. These are the elements that separate metals on the left side of the periodic table from non-metals on the right. Because these elements have properties of both metals and nonmetals, they are referred to as metalloids.

Some General Trends of Periodicity in Modern Periodic Table

Atomic Radius: It is the distance from the center of the nucleus to the valence shell of the atom. In Modern Periodic Table, Atomic Radius decreases as we go left to right in a period, and the atomic radius increases as we go down the group.

Ionization Potential: It is the ability of an atom to lose electrons from its valence shell. Higher Ionization Potential means difficult to lose electrons while lower ionization potential means easy to lose electrons. As we go down the group ionization potential decreases and as we move left to right in a group ionization potential increases.

Electronegativity: It refers to the ability of shared pairs of electrons by an element in a molecule. Electronegativity decreases as we go down the group and increases as we move left to right in a period. The most electronegative elements are F > O > N.

Metallic Character: Metals are elements that lose electrons and gain positive charge. The ability to lose electrons is given by ionization potential. Hence as we go down the group metallic character increases and as we move left to right in a period metallic character decreases.

Note: It should be noted that the above trend in properties is general in nature. There exist exceptions in between which we will discuss later.

FAQs on Periodic Classification of Elements

Q1: How many Elements are there in Periodic Table?

Answer:

There are 118 elements in Modern Periodic Table.

Q2: Why do we need Periodic Classification of Elements?

Answer:

As there are large number of elements it is difficult to remember properties of each element, hence,we need Periodic Classification of Elements to understand the properties in a more better and efficient way.

Q3: What are the Limitations of the Dobernier Triads?

Answer:

Dobernier Triad was valid for only 9 elements out of 30 elements available at that time. Also it was not valid for very high and low atomic masses of elements.

Q4: What is Modern Periodic Law?

Answer:

Modern Periodic Law states that the physical and chemical properties of elements is a function atomic number of elements and not atomic mass.

Q5: How does Atomic Radius vary in Periodic Table?

Answer:

In Periodic Table, atomic radius increases down the group and increases on moving left to right across periods.